Reticulocytes were measured on an automated reticulocyte counter in five groups; healthy adults, non-elderly patient with low hematopoiesis, adults with anemia, healthy elderly persons and elderly patients with anemia. The reticulocyte percent, absolute reticulocyte count, and reticulocyte composition as classified by fluorescence intensity (highly, moderately, and slightly fluorescent cells) were calculated. The healthy adults had a reticulocyte count of 0.70 +/- 0.55%, an absolute reticulocyte count of 4.36 +/- 1.90 X 10(4)/microliters, 2.33 +/- 1.95% highly fluorescent cells, 18.73 +/- 5.07% of moderately fluorescent cells, and 78.82 +/- 6.55% of slightly fluorescent cells. Patients with low hematopoiesis had lower counts except for the percentage of slightly fluorescent cell. Aged persons without anemia showed no differences in reticulocytes from healthy adults. However, elderly patients with anemia had a low reticulocyte count; there was no tendency towards an increase in the percentage of highly fluorescent cells or a decrease in the percentage of slightly fluorescent cells. Their reticulocyte percent was not significantly higher than healthy controls, suggesting that anemia observed in the aged arises from low hematopoietic activity in the bone marrow.